Bushing assemblies are generally located at a connecting joint between a base member and a member movable about the connecting joint in such applications as machinery, airplanes, boats and vehicular transportation. Generally, the base member has two ends, one end affixed to a frame and a second end having a mating member forming a part of a housing of the connecting joint. The member movable about the connecting joint also has two ends, one end affixed to an element movable with respect to the frame, and a second end having a mating member which along with the mating member of the base member forms the housing of the connecting joint. The bushing assembly is affixed within the housing of the connecting joint and serves to control forces and accommodate movement from the movable element.
Various forms of movements occur at the connecting joint including static and dynamic vibratory motions which induce radial and axial motions at the connecting joint. It is desirable that the bushing assembly be capable of damping such vibratory motions thereby reducing the transmittal of such vibratory motions to the base member while accommodating all static deflections.
Bushing assemblies are widely used in vehicular transportation such as joints in primary suspension assemblies for automobiles. One concern in automobiles is the reduction of vibrations induced from the road surface and isolation of the passenger compartment from such vibrations. Such vibrations may comprise a range of amplitudes and frequencies and motions in various directions. Relative to the road, the vibratory motion may be vertical, such as up and down motion induced by the road, or it may be horizontal, such as the sway motion incurred in cornering of a car. Automotive suspension systems are designed to reduce such vibrations. A front end suspension system is made up of components including various arms, rods, links, etc. intermediate of the frame and the wheel assembly of the car. Generally, an elongated arm extends from the wheel assembly, and another arm extends from the frame which are connected together at a connecting joint having a bushing assembly.
The most common type of bushing assembly is a rubber bushing. Rubber bushings generally comprise annular elongate inner and outer members with elastomer disposed therebetween. Such bushings are used to control and transmit movement but have limited capability in damping vibrations. Damping of vibrations is attained by dissipating the energy of the vibratory motion. The damping provided by elastomers is a function of the hysteresis property of the elastomer. In general, rubber bushings can be said to provide little damping.
One form of bushing assembly which can provide improved damping are fluid filled bushings. Fluid filled bushings generally include a cylindrical elongate inner rigid member, an elongate outer rigid sleeve member concentrically disposed and radially spaced from the inner member and a resilient means disposed between the inner member and outer sleeve member wherein the resilient elastomeric means defines a pair of circumferentially spaced and diametrically opposed fluid filled chambers fluidly connected by an elongate restricted passageway. In operation, in response to vibratory motions along the radial direction of the bushing assembly between the inner member and outer sleeve member, fluid is displaced from one chamber via the restricted passageway to the second chamber in a direction opposite to the vibratory motion. In particular, when a first chamber is contracted, the fluid is displaced therefrom through the restricted passageway to an expanding second chamber. In the reverse cycle of the vibratory motion, when the first chamber is expanding and the second chamber is contracting, the fluid is reversibly moved through the restricted passageway. As can be seen, an oscillatory motion of the fluid is generated within the restricted passageway between two chambers diameter about a radial direction.
The restricted passageway confines movement of the fluid. The oscillatory fluid in the restricted passageway creates a fluid resistance and/or a mass or inertia resistance to the pumping forces of the chambers resulting in damping of the vibratory motions along the radial direction. The chambers may be circumferentially spaced to provide damping along more than one radial direction in directions other than that which the chambers are located. Intermediate of the chambers, the bushing assembly comprises a solid rubber member, extending along the axial direction of the bushing assembly wherein these sections of the bushing assembly have the characteristics of the rubber bushing with respect to vibratory motion. Such fluid filled bushing assemblies provide damping limited to the radial direction along which fluid chambers are located. Damping is not provided along the axial direction of the bushing assembly.
An example of such a fluid filled bushing is disclosed in U.S. Pat. No. 3,642,268. The bushing there disclosed utilizes hydraulic fluid displaceable between two diametric chambers via a restricted orifice. The chambers are located in the bushing along a first radial direction whereas along a second radial direction perpendicular to the first radial direction is a solid rubber member which extends along the axial direction of the bushing. Such a fluid filled bushing exhibits low stiffness and high damping along the first radial direction dependent on the flow characteristics between the chambers and the fluid properties as described heretofore and high stiffness and low damping along the second radial direction and the axial direction.
Vibratory motions transmitted through bushing assemblies are not limited to motions in radial directions of the bushing but also include vibratory motions along the axial direction of the bushing assembly. Although the movement along the axial direction is controlled, the bushing assemblies described above provide limited damping of axial vibratory motions.
A fluid filled bushing for damping vibrations in both the radial and axial direction is disclosed in U.S. Pat. No. 4,667,942. The bushing therein disclosed utilizes hydraulic fluid displaceable between two sets of two chambers, the first set of two chambers provides damping in the axial direction and the second set of two chambers provides damping in the radial direction. The two sets of two chambers are fluidly interconnected via two restricted passageways. Vibratory motions in the radial direction are dampened by the transfer of fluid between the first set of two chambers via the two restricted passageways and vibratory motions in the axial direction are dampened by the transfer of fluid between the second set of two chambers via the two restricted passageways. Although this bushing assembly provides damping in the radial and axial direction, it is of complicated design. Furthermore, damping provided by such a bushing is diminished when the vibratory motion is changing from between the axial and radial directions.
There is a need for a fluid filled bushing assembly of less complicated design which can provide damping to vibratory motion in both axial and radial direction.